Zinc bacitracin composition for use as a feed supplement and method for making the same

ABSTRACT

A zinc bacitracin composition for use as a feed supplement which possesses improved stability to heat, mechanical mixing and moisture when used in substantially dry animal feeds. Zinc bacitracin is precipitated by the addition of zinc cations, as in the range of between 0.5 to 0.7 g Zn++/g bacitracin to a bacitracin fermentation broth, and then concentrated to 25 to 35% dry matter. Finely divided carrier material, such as calcium carbonate, is then added to the extent of 50 to 100 weight percent based on the dry matter, and the mixture is agitated to get a uniform mix. The uniform mixture is then spray dried to produce substantially spherical particles of the zinc bacitracin composition.

My invention relates to a zinc bacitracin composition for use as a feedsupplement, and to a method for making the same. The zinc bacitracincomposition of my invention possesses increased stability when admixedwith animal feeds. Such animal feeds containing the zinc bacitracincomposition of my invention may be thermally pelletized and/or processedunder conditions of high temperature and moisture, with relativelylittle decomposition of the zinc bacitracin.

BACKGROUND

Bacitracin is an antibiotic of relatively limited human therapeuticapplication, namely primarily topical usage. It is an excellent feedsupplement for disease prevention and treatment as well as a growthpromoter, and is extensively used in its relatively stable form, zincbacitracin, as an additive to livestock feeds, as for pigs and cattle,and to poultry feeds. Because of its limited human antibiotic utilityzinc bacitracin is to be preferred as an animal feed supplement forlivestock and poultry.

Zinc bacitracin has been developed as a feed supplement because of itsrelatively greater stability in the face of elevated temperatures andmoisture than bacitracin, see U.S. Pat. No. 2,809,892 issued Oct. 15,1957 to Francis W. Chornock. However, commercial feed grade zincbacitracin does not have an altogether satisfactory commercial stabilityto temperature and moisture. Efforts to improve the stability ofbacitracin compositions have included the addition of lignins to formcomplexes, see U.S. Pat. No. 3,035,919 issued May 22, 1962 to JackZiffer et al., or the addition of insoluble zinc salts such as zincoxide, see U.S. Pat. No. 3,025,216 issued Mar. 13, 1962 to Jack Zifferet al.

George Hines of Commercial Solvents Corporation of Terre Haute, Indiana,the manufacturer of the commercial feed supplement called "BACEFIRM",which is zinc bacitracin, has summarized the stability of this materialin an article entitled "Bacifirm (Zinc Bacitracin) In Liquid FeedSupplements" which was published in the AFMA Liquid Feed SymposiumProceedings by the American Feed Manufacturers Association, 53 W.Jackson Boulevard, Chicago Ill. 60604, in 1972.

Notwithstanding extensive research, the prevention of the decompositionof zinc bacitracin, particularly when used in conjunction with animalfeeds containing a variety of fats, vitamins, minerals, drugs such ascoccidiocides, etc. is not well understood. While there has been animprovement in the stability of commercially available feed grade zincbacitracin during the past decade, instability problems of thismaterial, particularly in the presence of temperature and moisture, andin the presence of widely used animal feeds, still exist.

I have determined that admixture of zinc bacitracin with animal feedspromotes the decomposition of the zinc bacitracin, particularly when theanimal feed-zinc bacitracin mixture is mechanically mixed and/or heated.

OBJECTS

This invention has as an object the provision of a zinc bacitracincomposition of enhanced stability in animal feeds, namely livestock andpoultry feeds, and particularly a zinc bacitracin composition which isresistant to adverse conditions of temperature, humidity, mechanicalmixing, and which may be used in animal feeds as a growth promoter andtherapeutic agent.

This invention has as another object the provision of a method forforming such zinc bacitracin composition having improved stability.

Other objects will appear hereinafter.

STATEMENT OF INVENTION

I have discovered that a zinc bacitracin compound with enhancedstability when mixed in feeds may be prepared by precipitating zincbacitracin from a bacitracin fermentation broth through the addition ofzinc cations in the ratios of about 0.5 to 0.7 g Zn⁺⁺ /g bacitracin*.Thereafter, I regulate the pH of the fermentation broth by theapplication of strong alkali, such as sodium hydroxide, to a final pH ofbetween 6.0 and 7.0. Thereafter, I remove the water from the medium byevaporation to a concentration on the order of 25 to 35 weight percentsolids, such as 30 weight percent solids. I then add between about 50 to100 weight percent based on the dry matter of a finely divided carrier,preferably calcium carbonate, and stir the slurry to obtain a uniformmixture. Thereafter, I spray dry the slurry to remove water to the orderof about 1 to 5 weight percent water, as on the order of 3 weightpercent**. The spray drying should be accomplished at an elevatedtemperature, such as the order of 300° to 450° C., and preferably 350°to 450° C., to effect very rapid drying. The resultant spray dried zincbacitracin composition in which the zinc bacitracin is intimatelyassociated with the carrier should be finely divided substantially solidparticles of high physical strength, namely relative freedom fromfracture and fines. At least 80 weight percent of the particles have asize of between 30 to 219 microns, with their shape being substantiallyspherical, and with their outer surfaces being relatively smooth. Amajor weight percentage of the particles should have a size of at least30 to 149 microns. Generally, the specific gravity of the particles isof the order of between 0.6 and 0.7 when CaCO₃ is added as the carrier.The calcium carbonate content of the spray dried product should rangebetween about 40 to 60 weight percent when the product contains 100grams of activity per kg based on an activity of 42 I.U. per milligram.Normally, without addition of CaCO₃ the particles have a hollow core,and are relatively prone to destruction, as by crushing.

I have determined that the zinc bacitracin composition of my inventionhas an enhanced stability when admixed with animal feed compositionsunder conditions of high temperature, mechanical mixing, or moisture.

I do not wish to be bound by any theory or mechanism relating to therelative stability of the zinc bacitracin composition of my invention,but I believe that its stability is due to a combination of factorswhich include the intimate association of the zinc bacitracin and thecarrier in the zinc bacitracin composition, the physical strength of theparticles, the substantially spherical shape of the particles, and therelative absence of very small fines and dust.

I have done comparative drying tests in which the carrier was omitted. Ihave determined that it is far easier to dry zinc bacitracin in thepresence of a carrier. Thus, in the absence of the carrier, the zincbacitracin tended to stick to the walls of the spray drier. The totalyield of active product is about 6 to 8% higher when the carrier ispresent. The flow properties of the product are far superior when thecarrier is present. Thus, the product flows easier and is easier tohandle. The product has a higher specific weight due to the carrierinducing the formation of substantially solid spheres having arelatively high mechanical strength.

I have determined that the yield of active product is about 10 to 15%lower when conventional drum drying is used in place of spray drying,and this I attribute mainly to thermal decomposition. The productderived from drum drying does not consist of spherical particles, andsuch product may be readily distinguished physically from the product ofthe present invention. The product from drum drying is not as stable inadmixture with animal feed compositions, particularly under conditionsof high temperature, mechanical mixing, and/or contact with moisture.

The carrier must be added to the concentrate prior to the spray drying.I have determined that adding finely divided calcium carbonate particlesto spray dried zinc bacitracin fermentation broth, with no addition ofthe calcium carbonate particles prior to spray drying, does not achievethe objects of the present invention. Thus, in such instance theaddition of the calcium carbonate merely functions as a diluent.

From comparative testing, such as the foregoing, I have concluded thatthe substantially spherical particles in which the zinc bacitracin is inintimate contact with the carrier are stable because of their relativelylow surface area, and because the carrier in some fashion shields thezinc bacitracin from the adverse affects of the contact with the animalfeeds and/or the adverse affects of undesirable ambient conditions, suchas increased temperature, mechanical mixing and/or moisture. At the sametime the activity of the zinc bacitracin in the animal feed is notadversely affected.

DETAILED DISCLOSURE OF INVENTION

The bacitracin fermentation media used by me are conventional and welldescribed in the literature. They may contain soybean oil meal,cornstarch, cottonseed oil meal, dextrose and mineral salts.

The fermentation of the bacitracin is achieved using conventionalmicroorganisms and conditions.

To achieve precipitation of zinc bacitracin, I follow substantially theprocedure described in Chornock U.S. Pat. No. 2,809,892 and use awater-soluble zinc salt, such as for example, zinc chloride, zincsulfate, zinc acetate, which I add to the nutrient medium in which hasbeen cultured a bacitracin-producing strain of Bacillus subtilis, nowdesignated Bacillus licheniformis, and which nutrient medium contains insolution bacitracin produced by the organism. Generally speaking, thebacitracin fermentation broth has a pH of about 8.2 to 8.4. Upon theaddition of the water soluble zinc salt, such as a solution of zincchloride having a concentration of 40 to 60 weight percent, such as onthe order of 50 weight percent, the pH drops to a range of about 5.0 to5.7, such as to about 5.5. The zinc bacitracin is then precipitated bythe addition of a solution of strong alkali, such as about 25 weightpercent sodium hydroxide, until a final pH of 6.0 to 7.0 is obtained.

I prefer to use higher concentrations of zinc than that disclosed inChornock U.S. Pat. No. 2,809,892. I prefer to use concentrations of zincwhich are in the range of 0.5 to 0.7 g Zn⁺⁺ /g bacitracin in thefermentation broth. The methods for microbiological extraction and assayfor the bacitracin are modified procedures of Grynne and Grove citedhereinafter in Footnote to Example 1.

The zinc bacitracin is precipitated from the solution, along with othermaterials which complex with zinc, such as proteins and peptides, whichare inevitably present in the medium. Water is then removed byevaporation using a conventional vacuum evaporator until there is aconcentration of about 25 to 35 weight percent dry matter.

When this concentration of dry matter is obtained, the finely dividedcarrier is then added, and the slurry is agitated by mechanical stirringto form a uniform mixture.

A variety of carriers may be utilized including calcium carbonate,calcium silicate, silica, calcium magnesium silicates, kaolin, magnesiumcarbonate, and other suitable carriers which may be present in animalfeeds without adverse effect on the animals ingesting such feeds, andwhich are capable of achieving an intimate association with zincbacitracin in the method of my invention.

The preferred carrier is calcium carbonate. The carrier should be veryfinely divided, and I prefer that the carrier should be sufficientlydivided so that at least 80 weight percent of the particles are below 40microns in maximum dimension. Preferably, about 85 weight percent of thecarrier should be of particles having a maximum dimension of below 20microns. Optimally, more than 50 weight percent of the carrier shouldhave a maximum dimension of less than 10 microns.

The amount of the finely divided carrier that is added to the slurry isof the order of 50 to 100 weight percent of the dry matter in theslurry.

A carrier which I have utilized is the commercially available finelydivided calcium carbonate trademarks "SJOHEST" (translation "SEAHORSE"),manufactured by Krithusbolaget i Malmo A.B. of Malmo, Sweden. Thiscalcium carbonate is derived from organic sea deposits and has aparticle size distribution of about 7 weight percent greater than 40microns; 16 weight percent greater than 20 microns; 27 weight percentgreater than 10 microns, and 44 weight percent greater than 4 microns.Its analysis is given in the following table:

    ______________________________________                                        Analysis of "SEAHORSE" Calcium Carbonate                                      Component        Weight Percent                                               ______________________________________                                        CaCO.sub.3       97.23                                                        MgCO.sub.3       0.30                                                         Fe.sub.2 O.sub.3 0.01                                                         Al.sub.2 O.sub.3 0.07                                                         SiO.sub.2        0.30                                                         Acid Insolubles  0.79                                                         Organic Matter   <0.36                                                        Water Soluble Matter                                                                           0.25                                                         Water            0.07                                                         MnO.sub.2         120 ppm                                                     ______________________________________                                    

The density of "SEAHORSE" is 0.8 (kg/l); the surface area is about 2square meters per gram BET, pH is about 9.0 to 9.2, the hardness isabout 3(Mho), and the crystal structure is cryptocrystalline.

The uniform mixture of the concentrated zinc bacitracin fermentationbroth and carrier is then spray dried. Conventional spray dryingequipment and techniques may be used to achieve a zinc bacitracincomposition of the present invention. However, I prefer to userelatively high inlet temperatures of air, such as on the order of 300°to 450° C., preferably 350° C. to 450° C., to achieve rapid drying. Whenrapid drying is achieved, there is a minimal amount of decomposition ofthe zinc bacitracin, since the relatively dry product can withstand hightemperatures and the water is removed very rapidly.

While as above indicated I prefer to dry to a water concentration of theorder of 1 to 5 weight percent, drying to other moisture levels may beaccomplished depending upon existing equipment facilities.

Typically, more than 80 weight percent of the dried product particleshave a dimension of between 30 to 219 microns, and preferably more thana major weight percentage of the particles should have a size of atleast 30 to 149 microns.

After drying, the zinc bacitracin composition may be blended with adiluent to standardize its potency, such as any of the conventionaldiluents like dried distiller's solubles, corn gluten feed, or soy beanmeal, and then blended with conventional animal feeds, such as poultry,swine or cattle feed.

The combination of the zinc bacitracin composition of the presentinvention with animal feeds possesses superior stability. The zincbacitracin-animal feed mixture may be treated at elevated temperatures,and stored with a smaller loss of potency than is achieved with existingmixtures of commercially obtainable zinc bacitracin and animal feeds.

The nature of the culture medium and the proteins and peptidesprecipitated with the zinc bacitracin does not appear to be significantin affecting the parameters and process variables of my process,although these materials may affect the stability of the zinc bacitracincomposition of my invention in some manner not understood by me. Theproteins and peptides of the culture medium may complex with the zinccations, and with the zinc bacitracin as in conventional zinc bacitracinmixtures used in animal feeds. Because of the presence of these otherzinc complexing materials it is not possible to specify the amount ofzinc that is complexed with the bacitracin in molar amounts.

The control of pH with strong alkali, such as sodium hydroxide, shouldbe such that the precipitate containing the zinc bacitracin should becompleted at a pH of between 6.0 to 7.0. The time of adjustment, rate ofagitation, and temperature of the solution during precipitation do notappear to be significant variables as long as a homogeneous masscontaining the zinc bacitracin is obtained.

EXAMPLES

The following examples are illustrative of the composition and processof the present invention. The examples are to be deemed as illustrativeand are not intended to delimit the particular proportions, materials,and other parameters which are set forth. Moreover, the specificationand the examples are addressed to one having skill in the art and it isnot intended that the same be distorted beyond reasonable limits.

The following experiments illustrate the effect of the concentration ofzinc on the stability of the zinc bacitracin in the fermentation broth.

EXAMPLE 1

Zinc chloride in various amounts is added to a fermentation broth with apH of 8.3 containing 700 IU/ml* under agitation (16.6 g bacitracin/l).After adjusting the pH to 6.5 to 6.8 the samples are heated in closedglass flasks at 90° C. for 4 hours and cooled at room temperature.

    ______________________________________                                        Zn.sup.++ g/l fer-                                                                       Bac.* g/l fer-                                                                             g Zn.sup.++ added                                                                         % loss                                    mentation broth                                                                          mentation broth                                                                            to 100 g bac.                                                                             of bac.                                   ______________________________________                                        0          16.6         0           52                                        2.4        16.6         14.5        40                                        4.8        16.6         29.0        35                                        7.2        16.6         43.0        35                                        9.6        16.6         58.0        26                                        ______________________________________                                         *Based on a potency of 42 IU per mg. The extraction of the zinc bacitraci     for potency measurement was affected by the procedure of Grynne set forth     in the article entitled "An Improved Method For The Determination Of          Bacitracin In Animal Feeds" in the May 1971 Analyst, Vol. 96, pages           338-342 at pages 338 and 339 by B. Grynne, with the modification that all     samples were treated with acetone. The zinc bacitracin in the extracts wa     determined microbiologically according to the procedure in "Assay Methods     Of Antibiotics", by D.C. Grove and W.A. Randall, Medical Encyclopedia Inc     (1965) pages 76 through 78, with the modification that the seed agar laye     was omitted. A 3-pont assay with the concentrations 0.01 to 0.05 and 0.02     I.U./ml for standard and sample was used.                                

EXAMPLE 2

The experiment was carried out with a fermentation broth containing 350IU/ml (8.3 g bacitracin/l). The procedure was analogous to thatdescribed in Example 1.

    ______________________________________                                        Zn.sup.++ g/l fer-                                                                       Bac. g/l fer-                                                                              g Zn.sup.++ added                                                                         % loss                                    mentation broth                                                                          mentation broth                                                                            to 100 g bac.                                                                             of bac.                                   ______________________________________                                        0          8.3          0           56                                        4.2        8.3          51.0        35                                        4.8        8.3          58.0        27                                        5.4        8.3          65.0        26                                        ______________________________________                                    

EXAMPLE 3

The experiment was carried out with a fermentation broth containing 420IU/ml (10 g bacitracin/l). The procedure was analogous to that describedin Example 1.

    ______________________________________                                        Zn.sup.++ g/l fer-                                                                       Bac. g/l fer-                                                                              g Zn.sup.++ added                                                                         % loss                                    mentation broth                                                                          mentation broth                                                                            to 100 g bac.                                                                             of bac.                                   ______________________________________                                        3.6        10.0         36.0        34                                        4.8        10.0         48.0        35                                        6.0        10.0         60.0        25                                        7.2        10.0         72.0        26                                        ______________________________________                                    

EXAMPLE 4

The experiment was carried out with a fermentation broth containing 630IU/ml (15 g bacitracin/l). The procedure was analogous to that describedin Example 1.

    ______________________________________                                        Zn.sup.++ g/l fer-                                                                       Bac. g/l fer-                                                                              g Zn.sup.++ added                                                                         % loss                                    mentation broth                                                                          mentation broth                                                                            to 100 g bac.                                                                             of bac.                                   ______________________________________                                        0          15.0         0           56                                        7.2        15.0         48.0        40                                        8.4        15.0         56.0        40                                        9.6        15.0         64.0        27                                        10.8       15.0         72.0        26                                        ______________________________________                                    

The following experiments reveal the spray drying of the zincbacitracin:

EXAMPLE 5

Zinc chloride in the amount of 0.5 g Zn⁺⁺ /g bacitracin was added to afermentation broth, thereafter the pH was adjusted to 6.8. Byevaporation the broth was concentrated and spray dried at air inlettemperatures of 350° C. to 400° C. and air outlet temperatures of 115°to 120° C. The recovery of the spray drying step in four individualexperiments was as follows:

    ______________________________________                                                             Dry matter of                                            Experiment                                                                             Temp. ° C.                                                                         concentrate, %                                                                             % recovery                                  ______________________________________                                        1        350         28           94.5                                        2        350         28           94.5                                        3        425         25           86.8                                        4        370         24           93.2                                        Mean recovery: 92.3%                                                          ______________________________________                                    

EXAMPLE 6

Zinc chloride in the amount of 0.50 g Zn⁺⁺ /g bacitracin was added tothe fermentation broth, thereafter the pH was adjusted to 6.7.

By evaporation the broth was concentrated. Based on its dry matter 50%resp. 100% "SEAHORSE" CaCO₃ was added. Subsequently spray drying wascarried out at air inlet temperatures of 350° to 400° C and air outlettemperatures of 115° to 120° C. The recovery of the spray drying step infour individual experiments was as follows:

    ______________________________________                                                Wt.                 Dry matter of                                                                           %                                       Experiment                                                                            % CaCO.sub.3                                                                            Temp ° C.                                                                        concentrate, %                                                                          recovery                                ______________________________________                                        5       50        350       28        101.5                                   6       50        400       28        102.5                                   7       50        400       25        98.2                                    8       100       420       25        96.1                                    Mean recovery: 99.6%                                                          ______________________________________                                    

    ______________________________________                                        Physical data of spray dried products.                                                      With CaCO.sub.3                                                                         Without CaCO.sub.3                                    ______________________________________                                        Particle size:                                                                         above 219 μ                                                                           approx. 10% approx. 10%                                            30 - 219 μ                                                                            approx. 80% approx. 80%                                            30 - 149 μ                                                                            approx. 60% approx. 60%                                            below 30 μ                                                                            approx. 10% approx. 10%                                   Moisture            1 - 3%      3 - 5%                                        Bulk density        0.6 - 0.7 g/cm.sup.3                                                                      0.3 - 0.35 g/cm.sup.3                         ______________________________________                                    

The following experiments demonstrate the stability of the zincbacitracin of the present invention in corn and feed mixtures:

EXAMPLE 7

The experiments were carried out in a feed mixer of total volume 54 l,using charges of 25 kgs corn. The mixer was provided with a steam jacketand prior to the addition of 200 ppm zinc bacitracin, the corn waspreheated to 95° C. During continuous mixing samples were drawn after 1,4 and 10 minutes. The samples were cooled by air before being analyzed.

In order to study their storage stability, samples were kept in paperbags at room temperature for 3 months before being analyzed.

The results are set forth in Table 1.

                  TABLE 1*                                                        ______________________________________                                        Maize Corn                                                                    Conditioning                                                                            loss in activity                                                                            during conditioning                                   time      Control product.sup.1                                                                       Modified product.sup.2                                ______________________________________                                         1 min.   0%.sup.3      0%.sup.3                                               4 min.   -11.7%        +4.5%                                                 10 min.   -29.9%        -4.5%                                                 ______________________________________                                         *With reference to Tables 1, 2 and 3:                                         .sup.1 Control product: drum dried product.                                   .sup.2 Modified product: spray dried product with addition of CaCO.sub.3      and 0.5 g Zn.sup.++ /g bacitracin before drying step.                         .sup.3 The analytical value of the 1 min. conditioned, not stored, sample     are used as reference (100% activity, i.e. 0% loss) for calculating           conditioning and total loss.                                             

EXAMPLE 8

The experiments were carried out in swine feed using the same conditionsas described in Example 7. Recipe of swine feed:

    ______________________________________                                        Components         Content %                                                  ______________________________________                                        Barley             38.00                                                      Ground corn        12.00                                                      Ground soybean, extr.                                                                            13.20                                                      Protein conc. for swines                                                                         10.00                                                      Rye bran           8.75                                                       Tapioca roots      8.00                                                       Luzerne meal       5.00                                                       Corn gluten feed   3.00                                                       Fat                1.00                                                       Mineral mixture    1.00                                                       Vitamin premix     0.05                                                                          100.00                                                     ______________________________________                                    

The results of the analyses on the swine feed are set forth in Table 2:

                                      TABLE 2                                     __________________________________________________________________________    SWINE FEED                                                                                              Loss during 3 months storage                                                                Total loss                            Conditioning                                                                         Loss in activity during conditioning                                                                      Modified                                                                           Control                                                                            Modified                         time   Control product.sup.1                                                                  Modified product.sup.2                                                                  Control product                                                                        Product                                                                            Product                                                                            Product                          __________________________________________________________________________     1 min.                                                                              0%.sup.3 0%.sup.3  -7.6%    -6.7%                                                                              -7.6%                                                                              -6.7%                             4 min.                                                                              -6%       -4.7%    Not analyzed                                                                           Not  Not  Not                                                                 Analyzed                                                                           Analyzed                                                                           Analyzed                         10 min.                                                                              -14.3%   -19.8%    -7.0%    +8.0%                                                                              -21.2%                                                                             -13.4%                           __________________________________________________________________________

EXAMPLE 9

The experiments were carried out in laying hen feed using the sameconditions as described in Example 7. Recipe of laying hen feed:

    ______________________________________                                        Components         Content %                                                  ______________________________________                                        Ground corn        40.00                                                      Ground wheat       5.00                                                       Ground soybean     15.00                                                      Corn gluten feed   11.75                                                      Wheat bran         11.00                                                      Meat and bone meal 5.50                                                       Luzerne meal       3.50                                                       Fodder chalk       3.00                                                       Fish meal          3.00                                                       Mineral mixture    1.00                                                       Fats               1.00                                                       Vitamin premix     0.25                                                                          100.00                                                     ______________________________________                                    

The results of the analyses are set forth in Table 3:

                                      TABLE 3                                     __________________________________________________________________________    LAYING HEN FEED                                                                      Loss in activity                                                                          Loss during                                                Conditioning                                                                         Control                                                                             Modified                                                                            Control                                                                             Modified                                                                            Control                                                                             Modified                                 Time   Product.sup.1                                                                       Product.sup.2                                                                       Product                                                                             Product                                                                             Product                                                                             Product                                  __________________________________________________________________________     1 min.                                                                                0%.sup.3                                                                            0%.sup.3                                                                           -7.8%                                                                               -5.8%                                                                               -7.8%                                                                               -5.8%                                    4 min.                                                                               -4.6%                                                                               +4.7%                                                                              -29.0%                                                                              -28.0%                                                                              -31%  -24.6%                                   10 min.                                                                              -20.9%                                                                              -11.4%                                                                              -36.2%                                                                              -27.7%                                                                              -50%  -32.7%                                   __________________________________________________________________________

By virtue of the improved stability against heat, mechanical mixing andmoisture possessed by the zinc bacitracin compositions of the presentinvention, animal feeds containing such zinc bacitracin compositionshave superior properties in terms of thermal treatments. Prior tothermally pelletizing animal feeds, it is conventional to condition suchanimal feeds by mechanical mixing of the feeds at a raised temperature,such as of the order of 75° to 85° C. Subsequent thermal pelletizationof animal feeds containing the zinc bacitracin compositions of thepresent invention may result in superior maintenance of the potency ofthe zinc bacitracin.

The shelf life potency of animal feeds containing the zinc bacitracincompositions of the present invention is superior to those of existingcommercial animal feeds containing zinc bacitracin.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification as indicating the scope of theinvention.

I claim:
 1. A process for the production of a zinc bacitracincomposition with enhanced stability suitable for use in animal feedswhich includes precipitating zinc bacitracin from a fermentation broth,adjusting the pH of the fermentation broth with alkali to within therange of 6.0 to 7.0, removing water by evaporation from the fermentationbroth without filtration to yield a slurry containing between about 25to 35 weight percent dry matter, adding about 50 to 100 weight percentbased on the dry matter of the slurry of a finely divided carrier inwhich 80 weight percent of the particles making up the carrier have adimension of below 40 microns, blending the mixture to uniformlydisperse the carrier particles, spray drying such uniform dispersion ata temperature of between about 300° to 450° C. so that the water israpidly removed to produce finely divided substantially sphericalparticles containing the zinc bacitracin in intimate contact with thecarrier.
 2. A process in accordance with claim 1 in which 80 weightpercent of the spherical particles have a dimension of between 30 to 219microns.
 3. A process in accordance with claim 1 in which the carrier iscalcium carbonate.
 4. A process in accordance with claim 3 in which amajor weight percent of the spherical particles have a dimension ofbetween 30 to 149 microns, and are substantially solid.
 5. A process inaccordance with claim 1 in which the amount of zinc cation that is addedto the bacitracin-containing fermentation broth to precipitate andstabilize the zinc bacitracin is between 0.5 to 0.7 g Zn⁺⁺ /g bacitracinin the fermentation broth.
 6. A process in accordance with claim 1 inwhich about 85 weight percent of the finely divided carrier particleshave a dimension of below 20 microns.
 7. A process in accordance withclaim 1 in which the spherical particles contain between about 1 to 5weight percent water.
 8. A process in accordance with claim 1 in whichthe spray drying is effected at a temperature of between 350° to 450° C.9. An animal feed additive made in accordance with the process ofclaim
 1. 10. An animal feed additive made in accordance with the processof claim
 2. 11. An animal feed additive made in accordance with theprocess of claim
 3. 12. An animal feed additive made in accordance withthe process of claim
 4. 13. An animal feed additive made in accordancewith the process of claim
 5. 14. An animal feed additive made inaccordance with the process of claim
 6. 15. An animal feed additive madein accordance with the process of claim
 7. 16. An animal feed additivemade in accordance with the process of claim 8.